1. Field of the Invention
The present invention relates to an active energy ray curable aqueous ink comprising a novel water-soluble polymerizable compound, and an ink-jet recording process, an ink cartridge, a recording unit and an ink-jet recording apparatus using this ink.
2. Related Background Art
There has heretofore been a technique that aqueous paints or inks are used in an active energy ray curing method comprising curing a resin composition in an ink by irradiation of light including active energy rays to form a cured film of a resin, thereby forming images. As for the material composition of the aqueous paints or inks used in this case, active energy ray curable resin compositions of a non-aqueous type are used, which are prepared into emulsions. Such an active energy ray curable resin composition in itself is not water-soluble. In such a technique, the hydrophilization of an ultraviolet curable resin itself or catalyst itself has been recognized as a problem to be solved. However, the hydrolization has not been realized under the circumstances.
Even in an ink-jet printing method, active energy ray curable inks are publicly known. However, resin compositions used in this case are also non-aqueous. As typical examples thereof, are known the so-called oil-based inks in which a pigment is dispersed in an organic solvent such as toluene or methyl ethyl ketone. Inks making no use of any solvent and composed of a monomer, oligomer or pigment dispersion are used in an application field of marking. These inks are unstable for precision printing because they are difficult to lower the viscosity thereof, and so they are exclusively used in dot printing such as marking for which precision is not required.
Such oil-based inks used in the ink-jet system as described above do not have ink properties suitable for high-quality full-color printing. More specifically, the oil-based inks are preferably used with care from the viewpoint of environment. In the case of an active energy ray curable ink using no solvent, it may be used only in printers great in dot and low in resolution. In U.S. Pat. No. 4,978,969, it is disclosed to use vinylpyrrolidone in a great amount like a solvent, thereby dissolving an ultraviolet curable adhesive therein to provide it as an ink for ink-jet recording. In this technique, it is intended to use it in thermal ink-jet apparatus. However, such technique has great restrictions as to materials in fact, is poor in general-purpose properties for reasons of, for example, using a monomer in a great amount, and can not be used in formation of high-quality images. The reason why the situation of use of the active energy ray curable inks is as described above is as follows.    1) An ink or paint using a resin dissolved in water tends to become insufficient even if it is cured in that, for example, the water fastness or gloss of an image formed is low.    2) Water-soluble active energy ray curable resins and water-soluble polymerization initiators (hereinafter also referred to as “catalyst” merely), which fulfill a catalytic function of aiding the curing of such a resin, are scarcely developed.
With respect to active energy ray curable aqueous inks, the following fact is further indicated as the cause that the material technique itself is hard to be advanced.    3) The use of water as a solvent poses a negative aspect from the viewpoint of practical use because water consumes a large quantity of energy for drying compared with organic solvents.
However, in fact, the active energy ray curing technique is expected as a curing technique that saves energy and is little in environmental pollution and environmental burden. The use of the active energy ray curing method is said to be useful in not only printing of images but also pretreatment for imparting printability to a printing base material and post-treatment for coating the base material printed with a material for protection or application in ink-jet printing.
Even to an application to ink-jet recording inks for full-color high-quality printing, hydrophilic materials such as the above-described water-soluble active energy ray curable resins and catalysts are extremely few under the circumstances. In order to apply them to an ink-jet system, materials that can be used in high-density nozzles and are low in viscosity and good in flowability are required. For example, resin materials (catalysts, polymerizable materials), by which the amount of a polymerizable compound added into an ink can be increased, actual drying time can be shortened, and the physical properties of an ink film cured in applying the ink to a recording medium followed by curing become excellent, and which have good compatibility with colorants, are required.
As first examples of such water-soluble active energy ray curable materials, for example, an ester of succinic anhydride and 2-hydroxyethyl (meth)acrylate, an ester of orthophthalic anhydride and 2-hydroxyethyl (meth)acrylate and vinylnaphthalenesulfonic acid are practically used as water-soluble polymerizable compounds having both acidic group and (meth)acryloyl group or vinyl group in a molecule.
As second examples, polymerizable compounds, to which hydrophilicity has been imparted by a polyethylene oxide chain, are known as compounds having at least 2 polymerizable functional groups in a molecule and industrially produced. Examples of such compounds include (meth)acrylic esters of polyhydric alcohols, such as diethylene glycol di(meth)acrylate and tetraethylene glycol di(meth)acrylate.
Japanese Patent Application Laid-Open No. H8-165441 discloses polyfunctional water-soluble polymerizable compounds. The compounds disclosed herein are obtained by a technique that water solubility is attained by increasing the number of hydroxyl groups in a molecule for imparting water solubility.
In Patent Documents, Japanese Patent Application Laid-Open Nos. H8-165441 and 2000-117960, (meth)acrylic esters of hydrophilic polyepoxides derived from a polyalcohol, and the like are practically used. These compounds are excellent in polymerizability by an active energy ray and physical properties of cured materials thereof, and the viscosities of aqueous solutions thereof meet a level required for ink-jet inks.
However, the compounds of the first examples may not become principal materials for water-soluble active energy ray curable materials because the number of polymerizable functional groups is one in a molecule, so that the rate of polymerization is slow, and the degree of crosslinking of cured materials thereof is markedly low.
According to an investigation by the present inventors, the compounds of the second examples have no water solubility when the length of the ethylene oxide chain is short, or involve a problem that physical properties of solids at the time they are polymerized or cured are often insufficient in performance required for paints or inks, such as hardness and adhesive property though the water solubility is achieved when the length of the ethylene oxide chain is long.
According to an investigation by the present inventors, the compounds in Japanese Patent Application Laid-Open No. H8-165441 are certainly excellent in polymerizability by an active energy ray and also in physical properties of cured materials thereof, but involve a problem that the viscosities of aqueous solutions thereof are somewhat high compared with the level required for ink-jet inks.
In Japanese Patent Application Laid-Open Nos. 2000-117960 and 2002-187918, when a hydrophilic dye soluble in water by an anionic functional group or an aqueous pigment dispersion that a pigment is dispersed in water by an anionic functional group is used as a colorant in an ink containing a water-soluble polymerizable material having a (meth)acrylate group, the pH of the ink is lowered to an acidic range with the formation of acrylic acid by hydrolysis of the (meth)acrylate group, whereby deposition of the hydrophilic dye stably dissolved in the ink at a pH of an alkaline to neutral range, or aggregation of the aqueous pigment dispersion stably dispersed at such a pH may occur in some cases, so that a problem arises from the viewpoint of long-term storability. In addition, in the case where a recording method is of an ink-jet recording system that thermal energy corresponding to recording signals is applied to generate ink droplets by the energy, the water-soluble polymerizable compound in the ink has been thermally polymerized by the thermal energy to form a polymer insoluble in water in a nozzle, thereby causing inconvenience in ejection of the ink.
It is accordingly a first object of the present invention to provide an active energy ray curable aqueous ink which is quickly polymerized by an active energy ray, high in crosslinking degree of a cured material formed therefrom and also excellent in adhesive property to recording media.
A second object of the present invention is to provide a water-soluble active energy ray curable ink which is quickly polymerized by an active energy ray, high in crosslinking degree of a cured material formed therefrom and also excellent in adhesive property to recording media, which ink is an active energy ray curable aqueous ink for ink-jet recording wherein the viscosity thereof meets the low-viscosity level required for ink-jet inks, and the ink has excellent long-term storability.
A third object of the present invention is to provide a water-soluble active energy ray curable ink quickly polymerized by an active energy ray, high in crosslinking degree of a cured material formed therefrom and also excellent in adhesive property to recording media, which ink is an active energy ray curable aqueous ink for ink-jet recording wherein the viscosity thereof meets the low-viscosity level required of ink-jet inks, and the ink has such an excellent long-term storability and excellent ejection stability that occurrence of thermal polymerization by thermal energy is prevented not to adversely affect the ejection of the ink even when a recording method is of an ink-jet recording system that thermal energy corresponding to recording signals is applied to generate ink droplets by the energy.
A fourth object of the present invention is to provide an ink-jet recording process, an ink cartridge, a recording unit and an ink-jet recording apparatus using this aqueous ink.
The above objects are achieved by the present invention described below.